Removes the named form item from the history stack (and from memory) if not currently shown.
Will return false when the form can't be removed, this can happen in certain situations:
1> The form is visible,
2> The form is executing a function (is actively used),
3> There are references to this form by a global variable/array,
4> If the form has a separate foundset with edited records that can't be saved (for example autosave is false)